A technical objective, that is also economically important, is to be able to make a cable comprising a twisted insulated metallic conductor pair or pairs as small as possible that is capable of transmitting data at a maximum rate. In order to provide a twisted pair cable being capable of transmitting digital signals at the highest rate for the maximum distance and also being as small as possible, insulating material with relatively low dielectric constant and low power factor is sought for the metallic conductor.
The advantages of relatively high bit rate transmission can be realized only if electrically balanced pairs can be produced. Pair balance means that one insulated conductor of a pair should be substantially identical to the other--a difficult objective. In addition to a good pair balance, maximizing both bit rate transmission and distance capability requires suitable crosstalk control. This carries with it a need for short pair twists which enhance the electrical characteristics of the pair as well as preventing the pairs from becoming untwisted.
Also desired is the ability to distinguish one conductor of a pair from another by sight. There is a basic conflict between the sight coding of insulated conductors and pair balance needed to provide electrically matched pairs. Sight coding involves making one insulated conductor of a pair appear differently from the other insulated conductor of the same pair. Striving for the required pair balance involves making one insulated conductor of a pair identical in every respect except appearance to the other conductor. The very best pair balances have been achieved with electrically matched pairs, i.e. the two insulated conductors of a pair taken successively from a single length of wire on the same insulating manufacturing line. Although electrically matched pairs produce the very best pair balance, the two resulting conductors have had the same color thereby making it impossible to sight distinguish between them.
Of importance with respect to colored insulation are electrical properties of cable which include such insulated conductors. One electrical property is capacitance. Capacitance is an effect somewhat similar to the magnetic field known to exist around a current-carrying conductor. The capacitive effect results from electrostatic charges on adjacent surfaces, such as metallic conductors in a pair or pairs. Electronic wires and cables by nature develop capacitive effects whenever current is flowing. Although it is impossible to eliminate capacitance, certain factors can be adjusted to achieve an acceptable level.
It is known that the inclusion of different colorant pigments in the composition of the insulation for purposes of distinguishing one conductor of a pair from the other compromises the electrical properties of the insulated conductor discussed hereinbefore. Conductor insulation which has a pigment dispersed throughout adversely affects electrical properties such as capacitance. Pigments of different color concentrates affect capacitance and processing differently. Achieving lower capacitance values has resulted in higher manufacturing costs whereas higher values cause increased attenuation.
The problems of the application of colorant materials to a moving insulated metallic conductor and of the effect of pigments dispersed throughout the insulation on electrical properties of the insulated conductor have been solved by the application of a colorant material to the surface of a moving insulated conductor which may be referred to as topcoating, for example. One such process is described in U.S. Pat. No. 4,877,645 which issued on Oct. 31, 1989 in the names of L. L. Bleich, J. A. Roberts and S. T. Zerbs. Relative motion is caused to occur between an insulated conductor and a source of a colorant material in a direction along a longitudinal axis of the insulated conductor. Colorant material is directed in spray patterns toward the insulated conductor in such a manner that substantially all the surface area of the insulated conductor is covered therewith. A first plurality of the spray patterns is such that each spray thereof occupies only an area of a plane and is at a predetermined angle to the longitudinal axis of the insulated conductor with the first plurality being disposed between a colorant supply head and a takeup. A second plurality of spray patterns may be disposed between the colorant supply head and a payoff. Each of the second plurality of spray patterns is fully conical. The first and the second pluralities of the spray patterns are arranged and spaced along the longitudinal axis of the insulated conductor.
Topcoating materially reduces scrap rates because the coloring is applied to the outside of the just-insulated conductor and therefore obviates the need to adjust insulating conditions for different colors and also the wasteful purging of an extruder for a color change.
With topcoating, it may be necessary first to tint the insulation with white color concentrates to hide the copper conductor. Here, it may be noted that copper wire can vary significantly from the familiar bright, shiny copper color to a dark, purplish brown. Because many desirable insulating materials are fairly transparent, providing a constant white base is helpful in achieving bright, easily distinguished colors. Placed on a white plastic material, for example, a topcoating satisfactorily produces readily distinguishable colors with acceptable adherence to the insulation and can be produced with acceptable processing yields.
Other processes for applying a colorant material to an outer surface of the insulation are known. For example, colorant material may be applied in periodic band marks around the circumference of the insulation or as a longitudinal stripe on the outer surface of the insulation.
The state of the art then is that there exist excellent materials which may be used for insulation as well as methods for causing these conductors to be identifiable. These materials and methods of coloring are advances in the quest for insulated metallic conductors which can transmit digital signals over long distances at the highest rate.
What is sought after and what seemingly is not provided for in the prior art is an electrically matched insulated metallic conductor pair in which the two insulated conductors of a pair are distinguishable. Desirably the matched pair is made from successive portions of a single length of metallic wire which is processed in sequential steps on an insulating line. Further what is sought after is a differentiation between the conductors of the pair without adversely affecting electrical properties of the insulated metallic conductors.